WIMPs as Candidates for Dark Matter

Summary

This project has two goals: First, to improve the prediction of the Dark Matter relic density, in order to match the anticipated reduction of the observational uncertainty due to upcoming data from the Planck satellite. To this end, leading radiative corrections will be calculated. Secondly, to analyse the impact of LHC data on the Dark Matter issue. These data will not only crucially test many models of particle Dark Matter; they may also allow to derive some constraints on the thermal history of the very early universe.

The Planck satellite started measurements of cosmic microwave background (CMB) anisotropies on 2009. This is expected to reduce the error on the inferred Dark Matter density to the level of one or two percent. More accurate theoretical calculations will be needed to match this reduced measurement error. This is the first topic to be addressed in this project.

Several well motivated extensions of the Standard Model of particle physics contain Weakly Interacting Massive Particles (WIMPs) which make good Dark Matter candidates. In the standard cosmology their relic density can be calculated from their annihilation cross section, under the assumption that all WIMPs were produced thermally. In this project, leading radiative corrections to this cross section will be calculated. These include corrections that are enhanced by large logarithms and/or large multiplicity factors, which can often be absorbed into running couplings; and corrections to quantities that vanish at tree level.

Well motivated theories containing WIMPs also predict the existence of additional new particles with masses near the TeV scale. The upcoming Large Hadron Collider (LHC) will therefore test these theories decisively. The second part of this project will investigate the bearing of LHC data on the Dark Matter issue, in particular on the identity and properties of Dark Matter particles.

If a positive signal for WIMP production (typically in the decays of heavier new particles) is found, the data may allow a first estimate of the WIMP annihilation cross section, and hence of the predicted WIMP relic density. A mismatch between this prediction and observation would require deviations from standard cosmology, or the introduction of a non-thermal production mechanism. Either way, LHC data lead to news insights into the evolution of the very early universe.

The initial focus of the work will be on supersymmetric theories, but this may change when LHC data become available. In the context of supersymmetry, the WIMP is the lightest neutralino, but the earliest LHC signals are expected to be due to the production and decay of squarks and gluinos. In the most optimistic scenario, LHC data will not only confirm the existence of superparticles with weak-scale masses, but also allow to deduce the most important WIMP annihilation channels, thereby reducing the effort required to derive accurate predictions of the relic density.